CHAPTER VII. THE MUSCULAR SYSTEM/
MUSCLES OF THE HEAD AND NECK,
THE platysma myoides (tig, *27A) is the sole representative of the cutaneous muscle which envelops the greater part of the body in lower Mammals. It arises from the superficial fascia over the pectoralis major and deltoid in the Chimpanzee and Gorilla. In
the Orang and Gibbon its origin extends to the mid-dorsal line. The muscles of both sides are continuous
in the middle line of the neck, and the fibres mav
decussate below the chin. Some fibres are inserted into the mandible, but most sweep into the face and blend with the labial muscles. When it contracts the lips are retracted and the teeth are exposed.
The occipito-frontalis varies individually, for the occipitales,frontales and epicranial aponeurosis differ in their relative sizes.
The orbicularis oculi (fig. 27A) consists as in Man of an orbital part surrounding the orbit and a palpebral part within the lids. These parts vary in size, and the former frequently sends a slip to the zygomatic mass. The orbital part arises from the nasal process of the maxilla and loops round the orbit; and the palpebral
* Variations iu human muscles are described in papers 7, 8, 11, 79, 133-